1. Field of Invention
The present invention relates to a method for forming a structure having a barrier layer and a glue layer above a polysilicon layer. More particularly, the present invention relates to a method for forming a titanium/titanium nitride (Ti/TiN) layer above a polysilicon layer functioning as a barrier layer and a glue layer for an overlying tungsten silicide (WSi.sub.2) layer.
2. Description of Related Art
FIG. 1 is a cross-sectional view showing a conventional MOS transistor gate structure. As shown in FIG. 1, the method of producing the MOS gate includes forming a gate oxide layer 2 over a semiconductor substrate 1, and then forming a doped polysilicon layer 3 over the gate oxide layer 2. Next, a tungsten silicide layer 4 is formed over the doped polysilicon layer 3. The doped polysilicon layer 3 and the tungsten silicide layer 4 together constitute a polycide layer, which acts as a conductive layer for the gate terminal. The doped polysilicon layer 3 is formed by a deposition using a low-pressure chemical vapor deposition (LPCVD) method followed by subsequent doping. The tungsten silicide layer 4 is also formed by depositing using a low-pressure chemical vapor deposition (LPCVD) method with tungsten hexafluoride (WF.sub.6) as the gaseous source for tungsten.
The tungsten suicide that reacts and forms over the polysilicon layer 3 has a chemical formula of WSi.sub.x, where the value of x is roughly between 2.6 to 2.8. The resistivity of the tungsten suicide layer is rather high, and is in the range of 700 to 900 .OMEGA.-cm. Because of the high value of x, the tungsten silicide is referred to as silicon-rich. In order to lower the resistance of the tungsten silicide layer, and hence reducing the resistance of the gate conductivity layer so that a high RC delay time is prevented, an annealing operation must be performed. After the annealing operation, the resistivity of the tungsten silicide is reduced to about 70 .OMEGA.-cm or below. Since the value of x in WSi.sub.x is now fallen to about 2.2 to 2.3, the tungsten silicide layer is now referred to as tungsten-rich. A 90% reduction in resistivity of the tungsten silicide layer due to annealing serves to lower the resistance of the polycide layer in the MOS gate considerably.
In the conventional method of forming a MOS gate polycide layer as described in FIG. 1 above, after a polysilicon layer 3 is formed over the gate oxide layer 2, the exposed portion of the polysilicon layer 3 will react in the presence of surrounding oxygen to form a layer of silicon dioxide known as a native oxide 5. The presence of a native oxide layer will lead to poor adhesion of the polysilicon layer 3 with the subsequently deposited WSi.sub.x layer. Furthermore, since the coefficient of expansion for a metallic thin film is many times greater than a silicon dioxide layer, peeling of the top WSi.sub.x layer as shown in FIG. 2 can happen after a high temperature processing treatment, for example, an annealing operation.
In addition, the deposition of tungsten silicide over the polysilicon layer 3 uses tungsten fluoride as the gaseous source for tungsten. Consequently, the tungsten silicide layer will contain some fluorine atoms. Later, these fluorine atoms can diffuse to the interface between the gate oxide layer 2 and the polysilicon layer 3, thereby affecting the effective thickness of the gate oxide layer 2.
In light of the foregoing, there is a need to provide a barrier layer for stopping the penetration of fluorine atoms as well as a glue layer for increasing the adhesion between a tungsten suicide layer and a polysilicon layer.